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Old 26th August 2014, 06:13 PM   #1
JFace is offline JFace  Canada
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Default Amplifiers that exceed grid leak resistance

I've seen quite a number of instrument tube amp designs that have a high mu gain stage driving a fixed bias pp stage using large grid leak resistors. Often times these resistors far exceed the max resistance indicated in the power tube data sheets.

Take for instance the Ampeg B15N. Using 6l6GC power tubes at fixed bias, the grid leak resistors should be no more than 50k, but the amps use 270k.

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What are the implications of amplifiers using such high grid leak resistances? Could designs such as this be improved upon by inserting a cathode follower pair between the inverter and the power tubes and reducing the grid leak resistors? Were the power stages designed with the expectation of bias drift?
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Old 26th August 2014, 06:41 PM   #2
DF96 is offline DF96  England
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High grid leak resistors can mean short valve life. A bit of gassiness leads to thermal runaway.

Two options:
- use a lower impedance drive stage which can cope with a smaller grid resistor value
- use a different output valve which is happy with a high grid resistance (e.g. EL34)
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Old 26th August 2014, 07:03 PM   #3
rayma is online now rayma  United States
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Quote:
Originally Posted by JFace View Post
I've seen quite a number of instrument tube amp designs that have a high mu gain stage driving a fixed bias pp stage using large grid leak resistors. Often times these resistors far exceed the max resistance indicated in the power tube data sheets.Using 6l6GC power tubes at fixed bias, the grid leak resistors should be no more than 50k, but the amps use 270k.
I've seen lots of Dyna Stereo 70s (which have 6CA7/EL34 output tubes and 270k Ohm grid resistors), but never had a problem with this.
Tubes in these amps can last a very long time. I have seen a few leaky coupling capacitors in these that caused some bias drift until they were replaced.
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Old 26th August 2014, 07:54 PM   #4
JFace is offline JFace  Canada
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Quote:
Originally Posted by rayma View Post
I've seen lots of Dyna Stereo 70s (which have 6CA7/EL34 output tubes and 270k Ohm grid resistors), but never had a problem with this.
Tubes in these amps can last a very long time. I have seen a few leaky coupling capacitors in these that caused some bias drift until they were replaced.
That would be within the spec of an EL34 in fixed bias mode (500k max).
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Old 26th August 2014, 11:29 PM   #5
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Most Guitar Amps abuse the max. Rg1 (grid leak) value. They do that to keep the loading on the phase splitter to a reasonable value so as to not lose too much gain.

The datasheet max Rg1 value applies to a tube running at the maximum dissipation that is listed on the datasheet.
If you bias the output tubes for a more typical 70% of maximum dissipation (at idle) then you can safely use 2 to 2.5 times the max Rg1 value.

Dissipation in the output tubes with signal will be greater than that 70% and it will increase as you overdrive more and more, but will still be sufficiently less that 100% of max. to be reasonable safe with Rg1 at the X2 Rg1 max datasheet value.

For guidance see this datasheet:
http://www.triodeel.com/kt88p1.gif
You willl note that as well as the different max Rg1 values for fixed and cathode bias there are also different values for when Anode plus Screeen dissipation is above or below 35 Watts. (Max Anode + Screen Dissipation is 42 + 8 = 50 Watts, so 35/50 = 70%).

Cheers,
Ian
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